Image intensifiers are well known for their ability to enhance night-time vision. The image intensifier multiplies the amount of incident light received by it to produce a signal that is bright enough for presentation to the eyes of a viewer. These devices, which are particularly useful for providing images from dark regions, have both industrial and military application. The U.S. military uses image intensifiers during night-time operations for viewing and aiming at targets that otherwise would not be visible. Night radiation is reflected from the target, and the reflected energy is amplified by the image intensifier. As a result, the target is made visible without the use of additional light. Other examples include using image intensifiers for enhancing the night vision of aviators, for providing night vision to sufferers of retinitis pigmentosa (night blindness), and for photographing astronomical bodies.
A typical image intensifier includes an objective lens, which focuses visible and infrared radiation from a distant object onto a photocathode. The photocathode, a photoemissive wafer that is extremely sensitive to low-radiation levels of light in the 580-900 nm spectral range, provides an emission of electrons in response to the electromagnetic radiation. This photo response is non-linearly related to the voltage at the photocathode (see FIG. 1, for example). Electrons emitted from the photocathode are accelerated towards a phosphor screen (anode), which is maintained at a higher positive potential than the photocathode. The phosphor screen converts the electron emission into visible light. An operator views the visible light provided by the phosphor screen.
Brightness of the image is increased by placing a microchannel plate (MCP) between the photocathode and phosphor screen. A thin glass plate having an array of microscopic holes through it, the MCP increases the density of the electron emission. Each electron impinging on the MCP results in the emission of a number of secondary electrons which, in turn, causes the emission of more secondary electrons. Thus, each microscopic hole acts as a channel-type secondary emission electron multiplier having a gain of up to several thousand. The electron gain of the MCP is controlled primarily by the potential difference between its input and output planes.
Two such image intensifiers tubes, the GEN II Image Intensifier Tube and a GEN III Image Intensifier Tube, are manufactured by ITT Electro Optical Products division, in Roanoke, Va. The GEN II Image Intensifier Tube employs an alkaline photocathode, whose potential varies roughly one volt. Depending on input light level, in the GEN III image Intensifier Tube, the photocathode is made of Gallium Arsenide. Unlike the alkaline photocathode of the GEN II tube, the Gallium Arsenide photocathode of the GEN III tube is susceptible to being bombarded by the positive ions from the MCP. To prevent this bombardment, the MCP is coated with a film of aluminum oxide.
A bright source can degrade the resolution of an image intensifier tube. Resolution of the tube is based upon its ability to resolve line pairs. When the tube goes to high light, the MCP increases the flow of electrons. Some channels in the MCP may become saturated, in which event resolution is degraded. If the source becomes brighter, the photocathode emits a greater number of electrons (i.e. the photocathode draws additional current). As a result of the MCP gain, more channels become saturated and the resolution is further degraded. The resolution of a bright source at high light becomes unacceptable.
Bright source protection circuits are employed to improve the resolution of an image at high light. In the GEN II tube, for instance, the photo response of the photocathode is reduced as the source becomes brighter. The bright source protection circuit includes a dropping resistor that is connected between the photocathode and a voltage multiplier, which provides an operating potential to the photocathode. As the current drawn by the photocathode increases, the voltage drop across the dropping resistor also increases. The potential supplied to the photocathode is lowered, and the photocathode provides a lower current in response to the bright input light. Thus, the photo response of the photocathode is automatically reduced and although the resolution is greatly reduced, the high light range of the GEN II image intensifier tub is increased.
This prior art bright source protection circuit cannot be employed for the GEN III tube. Whereas the voltage to the GEN II photocathode can be dropped to 1 volt out of 250, the voltage cannot be dropped to one volt for the GEN III photocathode. This is due to the aluminum oxide film on the MCP. Electrons emitted from the cathode must have sufficient energy to penetrate the aluminum oxide film; otherwise, the tube goes out. The voltage required to penetrate the aluminum oxide film is defined as the tube clamp voltage. Therefore, if the photocathode voltage is lower than the tube clamp voltage, the electrons from the photocathode cannot penetrate the aluminum oxide film, and the tube goes out.
To prevent the GEN III image intensifier tube from going out, the photocathode voltage is clamped at a level above the tube clamp voltage. The dropping resistor is connected between the voltage multiplier and the photocathode. The anode of a diode is connected to the input terminal of the photocathode, and the cathode of the diode is connected to a source that provides a power supply clamp voltage. The current drawn by the photocathode is increased until the cathode voltage reaches the power supply clamp voltage, whereupon the diode becomes forward biased. As a result, the cathode voltage is maintained at the power supply clamp voltage.
This circuit is difficult to implement in practice, however, since the tube clamp voltage is not always known. The tube clamp voltage is dependant upon the thickness and conductivity of the aluminum oxide film, which is dependant upon the manufacturing process. Thus, the thickness and conductivity varies with each tube. In a sample of GEN III tubes, the tube clamp voltage has a normal distribution curve with a mean of eighteen volts and a standard deviation of four volts. To avoid rejecting tubes during construction (i.e. to accommodate as many tubes as possible), the power supply clamp voltage is selected at 40 volts. If, however, the image intensifier tube has a tube clamp voltage of 10 volts, the photocathode will emit more electrons than the rest of the tube can handle. As a result, electrons pile up on the aluminum oxide film of the MCP and resolution at the phosphor screen is degraded. Thus, the problem of relying solely on the power supply clamp voltage--due to tube construction--is apparent.
Therefore, it is an object of the present invention to provide a bright source protection circuit that varies the photocathode voltage in response to current drawn by the photocathode.
It is a still further object of the present invention to provide a bright source protection circuit that pulse width modulates the photocathode voltage such that the tube is pulsed on and off.